A fin includes a blade that has an expanded area to produce enhanced water-kicking ability and propulsive force, so as to help a user, that is, a diver, to quickly move forward under water.
Generally, a fin has given specification and size. The propulsive force that can be produced by a fin is determined by the hardness of the material used to make the fin and the water-kicking force produced by the diver. Generally speaking, a fin having a blade made of a harder material tends to produce larger propulsive force. However, the diver must be strong enough to exert more strength to kick water with the harder blade. On the other hand, a blade made of a softer material tends to produce somewhat smaller propulsive force, and the diver may kick water with the softer blade without exerting too much strength. That means the diver may reserve more energy for swimming under water.
The blade of a fin is typically a solid flat piece, and has a performance decided by the material used to make the fin. However, the same type fin might have different users, and the same user or diver might need different fins for different diving environments or water activities. A conventional fin has given hardness that could not be changed once the fin is produced. That is, the conventional fin is not adapted to different users or different use conditions because it could not be changed in the hardness thereof, and accordingly the propulsive force that may be produced by it.
To solve the above problems, a fin with a blade having a plurality of lengthwise channels is developed. Elongated inserts may be separately inserted into the channels, so that the blade may have an overall hardness and accordingly a propulsive force adjustable by the number of the elongated inserts inserted in the channels on the blade.
The elongated inserts are separated from the fin and normally disorderly scattered when they are not inserted in the channels on the blade, and are therefore easily getting lost. Particularly, these elongated inserts are not always available for use when they are normally scattered, and not conveniently portable when they are temporarily removed from the channels on the blade. Although the above-mentioned elongated inserts are useful in adjusting the hardness of the fin, they are not convenient for use.
U.S. Pat. No. 5,746,631 discloses a fin having a left and a right blade, most part of which are separated from each other. However, since the left and the right blade are always in the separated state, they do not have the function of providing an adjustable closed area thereof.